1. Field of the Invention
The present invention is related to processes and compositions for repulping wet strength paper and paper products such as paperboard and the like, and in particular to repulping such materials which exhibit increased wet strength by virtue of having added thereto a wet strength resin.
2. Description of Background Information
Paper and paperboard are ubiquitous in our society, where these products are employed in numerous applications. Paper is an ideal material, not only for its functionalities, but also for its ability to be recycled. Its inherent recyclability is an important factor in today's market.
Paper is recycled via a process called repulping, wherein the cellulose fibers that comprised the original sheet are separated. These fibers can be cleaned, treated, redispersed, and prepared into a pulp slurry essentially similar to that used to make the original sheet. The normal papermaking process is then followed to form a sheet made from recycled fibers. The process of repulping involves mixing, under shear, in water. Chemicals may be added to accelerate the process; and elevated temperatures are often used.
Paper is made to provide specific functional properties. Chemicals are often added to impart and/or enhance these properties. Amongst the more widely used additives are wet strength resins. These chemicals act to provide strength to wet paper and are used in, among other paper products, paper towel and packaging.
Repulping paper containing a wet strength resin is difficult because the resin (such as a polyamide-epichlorohydrin resin) is added during paper production to enhance the strength of the paper produced so that the paper does not fall apart when used under wet conditions. The wet strength resin binds the cellulose fibers together, impeding the repulping process of separating the cellulose fibers. Typically, paper treated with wet strength resins will retain at least 15% of the dry strength of the paper when wet. Paper without wet strength resin generally retains only 2-7% of its dry strength when wet.
Oxidation facilitates the breakdown of the wet strength resin to permit separation of the cellulose fibers. Hypochlorite, particularly sodium hypochlorite, is typically used by paper mills in the repulping of wet strength paper to oxidize the wet strength resin to facilitate fiber separation. Hypochlorite oxidizes the wet strength resins within a narrow, carefully maintained pH range and within a temperature range of from about 122.degree. F. (50.degree. C.) to 158.degree. F. (70.degree. C.).
Environmental issues have been raised concerning the use of hypochlorite for repulping. These concerns relate to the formation of organic halides which are adsorbed by the pulp, chloroform emission, and the problem of adding chlorinated hydrocarbons to the effluent stream. For these reasons, non-halogen containing compounds, such as persulfates have been used to oxidize wet strength resin during the repulping process.
Thus, more recently it has been found that persulfate salts can be used to degrade the wet strength resin, facilitating the repulping process. Any persulfate salt, typically sodium, can be used. This material can also be used with alkali metal, alkaline earth metal, or ammonium salts of carbonate, bicarbonate or sesquicarbonate to enhance repulping performance. Mixtures of persulfate and carbonate, bicarbonate or sesquicarbonate can be prepared which do not separate in storage containers, and which exhibit substantially increased handling safety over persulfate alone.
In general, however, systems employing persulfate salts are slower than hypochlorite or similar systems. Moreover, the introduction of additional heat, beyond that used with hypochlorite, is generally required for repulping suspensions employing such persulfate salts, in order to accelerate the degradation of the wet strength resins to a commercially-acceptable rate.
WO 94/20682 A1 discloses a composition containing a persulfate and a carbonate, bicarbonate or sesquicarbonate for use in oxidizing wet strength resin containing paper. The present invention accelerates the action of this material.
U.S. Pat. No. 3,427,217 discloses a process for repulping wet-strength broke containing wet-strength resin which comprises slurrying the broke in a dilute aqueous solution of an inorganic oxidizing agent. The oxidizing agent is selected from a group which includes persulfates.
WO 95/05504 A1 discloses a formulation for repulping and/or decoloring broke comprising 51-94 wt % persulfate; 4-43 wt % pH adjuster; 0.05-6 wt % soluble catalyst selected from one or more of a soluble salt of Cu, Fe, Ag and Ni; and 0-10 wt % of a saccharide which has no greater ability to reduce the catalyst than does D-mannitol. This document also relates to a method for repulping and/or decolorizing broke. Preferably the pH adjuster is a base selected from alkali metal, alkaline earth metal or ammonium salts of carbonate, bicarbonate or sesquicarbonate. The catalyst is a Cu salt. The saccharide is ascorbic acid, D-sorbitol and/or D-mannitol. It is stated that the composition permits separation of the fiber without destruction of the cellulose, yet permits alerting the chromophore to decolorize. The composition is said to effectively meet environmental requirements. The pH range of the solutions of Example 1 are relatively high, and the control of pH by the use of a buffer or other means is not disclosed.
THORP, et al., "Chlorine-Free Wet-Strength Paper Repulping and Decolorizing with Activated Persulfates", 1995 Papermakers Conference: Proceedings (TAPPI); 163-168 (Apr. 26, 1995; TAPPI Press) discloses a sodium persulfate mixture containing an activated persulfate catalyst used in laboratory trials carried out on wet-strength broke that investigated the use of activated persulfates for repulping and decolorizing papers treated with neutral/alkaline wet-strength resins. Reaction time, persulfate concentration, temperature, and pH were identified as the critical process parameters in optimizing repulping performance. Paper additives, lignin content, dye chemistry, and mechanical agitation were also found to affect persulfate performance. Page 165 discloses that wet strength broke is most effectively repulped by activated persulfates under alkaline conditions. This page also states that a pH range of 9-11 usually yields the best repulping results.
It is known that the ratio of persulfate to base can be adjusted to provide a neutral, acidic, or basic pH during the repulping process for the oxidation of the wet strength resin. Such pH regulation by adjustment of the persulfate to base ratio in the combined product avoids the need for an additional process step for pH adjustment and permits a single package chemical treatment for repulping. Repulping using persulfate is conducted under alkaline conditions, because conventional wisdom dictates that repulping using persulfates is optimal, for example faster, under alkaline conditions.
For Example, ESPY, et al., "Persulfates as Repulping Reagents for Neutral/alkaline Wet-strength Broke", Tappi J., Volume: 76 Number: 2, pages 139-42 (1993) CA: 119(10)98260h discloses alkali metal persulfates have been found to repulp neutral/alkaline wet strength broke. Persulfate salts defiber paper made with polyamide-epichlorohydrin resins, although slower than hypochlorite salts. Persulfate salts are said to be an acceptable substitute for hypochlorite because they do not generate organic chlorides in pulping effluents. Persulfate salts are especially effective in broke made with polyamide- or polyamine-epichlorohydrin wet-strength resins. Temperature and pH conditions are also described.
ESPY, et al., Proceedings of the TAPPI Papermakers Conference--1990, pg. 147-149, discloses that repulping is much faster under alkaline and neutral conditions than acidic conditions (pH 4). The pH of the suspension or system appears to vary considerably throughout the process and is not held or maintained substantially constant throughout the process. H. H. Espy and G. W. Geist, TAPPI Journal, pp. 139-142, Vol 76, Nov. 2, 1992, which is a related version of the foregoing, presents similar disclosure.
WO 95/06157 A1 discloses a process for repulping wet strength paper in which the initial breakdown is conducted at one pH; with substantial completion at a second, and high pH. The first pH value is no higher than 8, but is said to be typically about 8 (page 4, line 25), and preferably in the acidic range, and the pH adjusting agent for the first pH value is said to be typically an organic acid such as acetic acid, or an inorganic acid such as hydrochloric acid or sulfuric acid. The pH may be as low as 3 (Table 1, page 8) or 4 (Table 2, page 11, and Table 3, page 13). The pH is not controlled by a buffer solution, and the efficacy is undesirable.
U.S. Pat. No. 5,447,602 (which is a family member of WO 95/06157 A1 discussed above) discloses a process for repulping wet strength paper containing a wet strength resin. The process comprises (a) repulping the wet strength paper in a slurry containing an oxidizing agent and having a first pH to initiate breakdown of the paper to fibers; and (b) repulping the slurry at a second pH which is higher than the first pH until conversion of the wet strength paper is substantially complete. The process is said to be especially useful for repulping bleached paper and unbleached paper broke or corrugated containers.
EP 585 955 A1, a patent application of Hercules Incorporated, discloses the use of a non-chlorinating oxidizing agent and a water soluble buffering salt that is capable of maintaining pH between 7 and 12.
THIRKETTLE, "Broke Handling", Paper Review of the Year, Johnsen, Jorgensen & Wettre, Ltd., 1974 (pp. 126-136) discloses various aspects of handling "broke" (i.e., paper trimmings, scrap, and similar surplusage resulting from paper manufacture), including broke repulping. This document specifically teaches that repulping of broke treated with agents such as KYMENE 557 can be most efficiently accomplished under alkaline conditions at a pH of 10.5 and a temperature of 120.degree. F.
Each of the foregoing approaches suffers from disadvantages. For example, repulping using persulfates and similar agents is generally slower than repulping with hypochlorites, and so the use of persulfates also includes a sacrifice in speed, efficacy and efficiency.
Accordingly, the foregoing approaches have failed to provide processes and compositions which exhibit the desired degree of effectiveness, rapidity and simplicity, and there has been a continuing need for improvement.